1. Field of the Invention
This invention relates generally to a process for the production of phenolic compounds from coal. In another aspect, this invention relates to a process for preparing char and gaseous liquid fuel products from coal. More particularly, this invention concerns a method of reacting coal with hydrogen in a manner such that the ratio of phenolic compounds produced to hydrogen consumed is maximized. More particularly, this invention also relates to a continuous hydrocarbonization process employing a fluid-bed reaction zone for converting coal to char and gaseous and liquid fuel products.
2. Description of the Prior Art
Increasing energy needs have focused attention on solid fossil fuels due to their availability in the United States in a relatively abundant supply and their potential value when converted into more useful forms of energy and feedstock. Coal is known to be a potential valuable source of chemical compounds as well, and considerable effort has been expended in an attempt to develop a process for the efficient production of such chemicals and such fuel products. The first processes involved the carbonization of coal in an inert atmosphere to produce only about 5 to 15 weight percent generally about 10 to 15 weight percent based on the coal charged, of liquid product and about 70 to 75 weight percent of a solid char. Since the products were generally suitable only as fuels, these processes were not commercially feasible in this country. The low yield and poor quality of products rendered them commercially unattractive. The worth of the unit heating value of the solid char product even with all the gas and liquid product was less than that of the coal charged.
In an effort to convert the bulk of the coal to a liquid product, the hydrogenolysis processes were developed. In these processes a recyclable "pasting oil" was necessary to initially dissolve or slurry the raw coal; the slurry of coal and usually a catalyst in oil was heated in the presence of hydrogen gas at 450.degree. C to 550.degree. C and about 2000 to 10,000 p.s.i.g., generally 5000 to 10,000 p.s.i.g.; and up to 20 to 30 percent of the finely-divided unreacted coal and ash had to be filtered off or otherwise removed from the heavy, viscous primary oil product. Although these processes were successful in that the amount of liquid products were substantially increased, they were not commercially acceptable because the investment, the operating costs and in particular the hydrogen requirements were too high in comparison with the value of the products obtained. They are considered only in special economic conditions where alternate energy sources such as crude oil are expensive or unavailable.
More recently, dry "hydrocarbonization" processes were developed wherein coal was heated with hydrogen gas. However, these processes were generally batch-type processes and, because they were conducted at greatly elevated temperatures and pressures, resulted in the production of hydrocarbon gases and liquids useful mainly as fuels. Moreover, these batch processes were not convertible to operable continuous processes in any obvious manner. Greatly elevated temperatures and pressures at which these processes functioned also make them difficult to operate and impractical. It was shown in U.S. Pat. No. 3,231,486 that a sub-bituminous coal, Elkol coal, may be carbonized under mild operating conditions in the presence of hydrogen in a fluid-bed. Other processes were directed toward total gasification rather than the production of both gas and oil.
Total gasification requires large consumption of hydrogen as well as difficult and costly operating conditions. For example, using the crude stoichimetric equation, CH.sub.8 (coal) + 1.6H.sub.2 CH.sub.4, as a basis for roughly calculating hydrogen consumption, total gasification of 100 pounds of idealized coal (CH.sub..8) at 100 percent efficiency would require 25 pounds of hydrogen. This is a hydrogen consumption of about 25% of the coal by weight. The hydrogen could be supplied, for example, by steam gasification of an additional 57 pounds of idealized coal (CH.sub..8), and the consumption of an additional large quantity of coal, depending on the process, as fuel.
The object of this invention is an improved process for the hydrocarbonization of coal wherein the primary products comprise a mixture of both gaseous and liquid products and wherein the process consumes modest amounts of hydrogen amounting to about 1 to about 5 weight percent of the coal charged. By the term hydrocarbonization as employed throughout the specification, is meant a pyrolysis or carbonization in a hydrogen-rich atmosphere under such conditions that significant reaction of hydrogen with coal and/or partially reacted coal and/or volatile reaction products of coal occurs.
The hydrocarbonization process of this invention provides improved control over product yield, quality and distribution. Although product distribution between gas, liquid and solid carbonaceous residue is to a certain extent a function of the nature of the particular coal charged, the pattern may be altered considerably by variation in reaction conditions such as pressure, temperature, residence time and type of recycle operation used. Moreover, regardless of the yield and/or distribution, as a result of hydrocarbonization, the end products are also more stable than those obtained from the same coal by pyrolysis.
The process of this invention is an improved hydrocarbonization process wherein the primary product, amounting up to about 5 to 10 weight percent of the coal charged, consists of valuable phenolic compounds. In addition, the ratio of phenolic compounds and other liquid products to the amount of hydrogen consumed is considerably higher than that of the prior art process, resulting, for the first time, in an economically attractive method for obtaining chemicals, particularly phenolic compounds from coal. Moreover, when liquid and fuel products are desired, the hydrocarbonization process of this invention also provides a ratio of liquid and gaseous fuel products compared to the amount of hydrogen consumed that is considerably higher than that of prior art processes, resulting in an economically attractive process for converting coal to liquid and gaseous fuel products.
In addition, the amount of char is reduced from 70 to 75 percent to less than 60 percent, and often as low as 30 percent of the coal charged. More significantly, the improved control over product yield, quality and distribution in the hydrocarbonization process of this invention makes it particularly adaptable and integratable into an essentially internally balanced process. Conversion to liquid and gas products may be controlled to produce an amount of char lust sufficient to satisfy other supportive needs, such as hydrogen production, plant fuel and miscellaneous high-level, heat energy requirements.
The process of this invention, in its broadest aspect, comprises continuously feeding particulate coal and a hydrogen-containing, oxygen-free gas to a hydrocarbonization zone under relatively mild conditions of temperature and pressure to convert said coal to a vapor and a solid char, and continuously withdrawing the vapors and char from the hydrocarbonization zone. In this process, there is a continuous movement of the solids in the fluidized bed throughout the hydrocarbonization zone, with the composition of solids in the bed approximately that of the char.
The process of this invention, broadly stated, also comprises continuously fluidizing a dense phase flow of coal particles in a finely-divided form; preheating the fluidized particles in an essentially oxygen-free atmosphere to a temperature below the range where surface plasticity, or stickiness is developed; introducing the preheated particles into the bottom of a hydrocarbonization zone at a high velocity; fluidizing the coal, partially reacted coal and char particles as a fluid-bed in the zone with a hydrogen containing, oxygen-free gas; reacting the coal particles with hydrogen under relatively mild conditions of temperature and pressure to convert the coal particles to a vapor and a solid char; and continuously withdrawing the vapors and char from the hydrocarbonization zone. In this process as well, there is a continuous movement of the solids in the fluidized bed throughout the hydrocarbonization zone, with the composition of the solids in the bed approximately that of the char.
The reaction products are hydrocarbon gases, mostly saturated, non-hydrocarbon gases, principally carbon monoxide and carbon dioxide, light hydrocarbon liquids, tar, water and char. The tar reaction products contain high concentrations of phenolic compounds, aromatic hydrocarbons and precursors, and gasoline components or precursors. When desirable, the tar reaction products are readily convertible to hyrocarbon fuel products by methods well know to those skilled in the art, such as hydrotreating. Tar yields in this hydrocarbonization process are at least approximately double those of carbonization in the absence of hydrogen. Furthermore, the tar yields may be controlled over a range by varying reaction conditions such as time, pressure and temperature.
It has been discovered that exposure of coal to oxidizing conditions during the various phases of operation reduces tar yields upon hydrocarbonization of the coal. In order to maximize the production of phenolic materials from coal, non-oxidizing conditions or substantially non-oxidizing conditions must be employed in all phases of the operation such as during the mining, shipping, storage, preparation and reaction of the coal employed especially where the lowest rank coals are used such as sub-bituminious coals particularly those of the non-agglomerating type especially types such as sub-bituminous C and lower ranked coals such as lignitic coals. In general, therefore, substantially non-oxidizing conditions should be employed in mining, shipping, storage, preparation and in the process itself when it is desirable to maximize tar yields. On the other hand, it is recognized that a limiting preoxidation may be beneficial to reducing the agglomerating tendency of certain other coals such as, for example, agglomerating, high-volative A, bituminous coals. In this case, a trade-off exists between loss of desirable products and reduced agglomeration.
Coal has been classified according to rank as noted in the following table, Table A.
Table A __________________________________________________________________________ Classification of Coals by Rank..sup.a (Legend: F.C. = fixed carbon; V.M. = volatile matter; B.t.u. = British thermal units) __________________________________________________________________________ Limits of fixed carbon Class Group or B.t.u., ash free basis __________________________________________________________________________ 1. Meta-anthracite Dry F.C., 98% or more (dry C.M., 2% or less) 2. Anthracite Dry F.C., 92% or more and less than 98% (dry V.M., 8% or less and I. Anthracite more than 2%) 3. Semianthracite.sup.b Dry F.C., 86% or more and less than 92% (dry V.M., 14% or less and more than 8%) 1. Low-volatile bitumi- Dry F.C., 78% or more nous coal and less than 86% (dry V.M., 22% or less and more than 14%) 2. Medium-volative bitu- Dry F.C., 69% or more minous coal and less than 78% (dry V.M., 31% or less and more than 22%) II. Bituminous.sup.d 3. High-volatile A bitu- Dry F.C., less than 69% minous coal (dry V.M., more than 31%) 4. High-volatile B bitu- Moist.sup.c B.t.u., 13,000 or minous coal more and less than 14,000.sup.e 5. High-volatile C bitu- Moist B.t.u., 11,000 or minous coal.sup.f more and less than 13,000.sup.e 1. Sub-bituminous A coal Moist B.t.u., 11,000 or more and less than 13,000.sup.e III. Sub- 2. Sub-bituminous B coal Moist B.t.u., 9,500 or more bituminous and less than 11,000.sup.e 3. Sub-bituminous C coal Moist B.t.u., 8,300 or more and less than 9,500.sup.e 1. Lignite Moist B.t.u., less than IV. Lignitic 8,300 2. Brown coal Moist B.t.u., less than 8,300 __________________________________________________________________________ .sup.a This classification does not include a few coals that have unusual physical and chemical properties and that come within the limits of fixed carbon or B.t.u. of the high-volatile bituminous and sub-bituminous ranks All of these coals either contain less than 48% moisture and ash free fixed carbon or have more than 15,500 moist, ash free B.t.u. .sup.b If agglomerating, classify in low volatile group of the bituminous class. .sup.c Moist B.t.u. refers to coal containing its natural bed moisture bu not including visible water on the surface of the coal. .sup.d It is recognized that there may be noncaking varieties in each group of the bituminous class. .sup.e Coals having 69% or more fixed carbon on the dry, mineral-matter-free basis shall be classified according to fixed carbon, regardless of B.t.u. .sup.f There are three varieties of coal in the high-volatile C bituminou coal group, namely, Variety 1, agglomerating and non-weathering; Variety 2, agglomerating and weathering; Variety 3, nonagglomerating and non-weathering. Source: A.S.I.M. D388-38 (ref. 1).
Referring to Table A above, the preferred coals when it is desirable to maximize the yield of tar reaction products quantitatively such as phenolic compounds according to the process of this invention, comprise the lowest ranked coals, the nonagglomerating, sub-bituminous and lignitic classes, III and IV.
For purposes of definition, the non-oxidizing conditions as used to describe and claim the invention refer to any condition of mining, transportation, storage, drying and reacting the coals, especially the preferred coals employed according to this invention, the lowest ranked coals, that allows for between 80 to about 99 percent especially about 90 to 99 and preferably about 95 to about 99 percent of maximum production of phenolic compounds or tar reaction products in general, employing the other enumerated and claimed reaction condition according to this invention. Maximum recovery or manufacture of phenolic compounds or other tar reaction products such as aromatic hydrocarbons and gasoline precursors, employing the reaction conditions of the present invention is based on the phenolics or other tar reaction products recovered from the coal especially the preferred coals of the present invention, the lowest ranked coals, which are at no times exposed or substantially exposed to any air or other oxidation conditions prior to hydrocarbonization.
The coal employed in the process of this invention can be any coal which is non-agglomerating under the process conditions, such as the lignites, sub-bituminous C coals and the like. Preferred non-agglomerating coals are those containing at least 15 percent oxygen, and preferably 18 to 25 percent oxygen, on an MAF basis.
Lightly to moderately agglomerating coals may be used in the process of this invention without a separate pretreatment step added to prevent agglomeration of coal particles in the fluid-bed hydrocarbonization zone. Ordinarily, such a pretreatment would be necessary in a continuous hydrocarbonization process since even those coals considered to be non-agglomerating coals such as lignites or coals from certain sub-bituminous seams are susceptible to agglomeration and tend to become sticky in a hydrogen-rich atmosphere. Moreover, feeding heavy liquid materials to the fluid-bed hydrocarbonization zone is known to cause de-fluidization of the bed due to particle agglomeration and plugging. Such heavy liquid materials may be recycled heavy tar products to be converted to lower molecular weight products, light liquids and gases. Or they may be heavy liquids from an external source which have been added to enrich the normal gas and/or liquid product, or as a means of waste disposal. However, according to the process of this invention, a separate pretreatment is not necessary for the handling of the lightly to moderately agglomerating feed materials.
Such feed materials may include the low rank coals, such as lignites and sub-bituminous C coals even some moderately agglomerating bituminous coals and also recycle product liquids. Preferred coals which may be used according to the process of this invention without any pretreatment step added to prevent agglomeration comprise the lowest ranked coals, the non-agglomerating, sub-bituminous and lignitic classes, III and IV of Table A above, the non-caking bituminous coals referred to in Table A and a few moderately agglomerating or caking-coals.
More highly agglomerating coals, such as most bituminous coals, are strongly agglomerating in a hydrogen atmosphere. They can not be handled conventionally even with a pretreatment step. These coals may now be handled without an injurious degree of defluidization by the process of this invention alone or in combination ith a pretreatment step, if necessary. If a pretreatment step is necessary, the needs for pretreatment are milder and cost less. For example, even after heavy conventional pretreatment, the use of a highly agglomerating coal such as Pittsburgh Seam Coal, in a hydrocarbonization process, presents the problem of agglomeration occurring in the fluid-bed hydrocarbonization zone. However, it is beneficial to use the process of this invention to overcome this agglomerating problem.
Those skilled in the art will recognize that any number of suitable pretreatment steps may be applied in combination with the process of this invention for the handling of coals which are either highly agglomerating or highly agglomerating in a hydrogen-rich atmosphere. These pretreatment steps include, for example, but are not limited to, chemical pretreatment, such as oxidation, or mixing with inert solids such as recycle char. It should be noted, however, that when coal is subjected to an oxidation type pretreatment to prevent agglomeration in the hydrocarbonization zone, the oxidation of the coal also results in a quantitative loss in the maximum realizable amount of tar product.
Coal particles in a particulate state may be used in the process of this invention. The coal size can be about 8 mesh or less, with particle sizes of less than about 20 mesh being preferred. There is no need to remove very fine particles, but it is desirable to minimize them by appropriate selection of the grinding process.
According to the improved process of this invention, the coal particles are preheated before entering the hydrocarbonization zone. The coal particles are in a dense phase flow. By dense phase as used throughout the specification is meant a concentration of solids in fluidizing gas of from about 5 pounds to about 45 pounds of solids per cubic foot of gas more typically from about 15 pounds to about 40 pounds of solids per cubic foot of gas. A dense phase of coal particles should be distinguished from the dilute phase wherein the concentration of solids in fluidizing gas is typically from about 1 pound to about 2 pounds of solids per cubic foot of gas. In coal conversion processes employing a dilute phase flow of coal particles, preheating steps have generally involved passing the coal particles around hot pipes or using large quantities of hot gases to impart heat to the coal particles directly. Indirect heat transfer in coal conversion processes employing a dilute phase flow of coal particles is uneconomical and impractical due to the inherently poor heat transfer coefficients of the pipelines in dilute phase flow, approximately 1 BTU to 2 BTU per hour per .degree. F per square foot of inside surface area of the pipeline. However, it has been found that a dense phase flow of coal particles may be conveniently and economically preheated by indirect heat transfer means.
In the hydrocarbonization process of this invention, the flow of coal particles in dense phase provides the following benefits. The quantity of coal transferred and heated per unit of pipe cross-sectional area not only exceeds that obtainable in dilute phase flow but also uses less power. A cubic foot of gas conveys 15 to 30 times more coal particles in dense phase flow than in dilute phase flow. The use of a comparatively small amount of conveying gas in dense phase flow may be extremely beneficial downstream if, for example, flue gas or nitrogen gas is used as the conveying gas. Large amounts of conveying gas other than hydrogen-rich gases or recycle gas are undesirable in the fluid-bed hydrocarbonization zone and must be separated from the coal particles before entering the fluid-bed by suitable equipment such as a cyclone separator or the like. Moreover, if such a separation is desired, in dense phase the coal particles are more easily separated from the conveying gas before entering the fluid-bed hydrocarbonization zone. Also, power requirements are intrinsically smaller in dense phase flow due to lower carrier gas velocities. In dilute phase flow, the linear velocity of carrier gases is between 50 and 100 feet per second to prevent entrained coal particles from settling out in pipelines. However, in dense phase flow, the linear velocity of carrier gases may be about 20 feet per second and sustain steady flow in the pipelines.
According to the process of this invention, a dense phase flow of coal particles is preheated by indirect heat transfer means to a temperature up to about 420.degree. C. For example, a dense phase of coal particles may flow through a multiplicity of parallel pipelines which are externally heated. The heat transfer coefficient of the pipeline has been found to approximate that found in heat transfer through the walls of a fluid-bed, about 20 to about 40 BTU per hour per square food of inside surface area per .degree. F. The externally heated pipelines which the coal particles pass through are heated to a predetermined temperature sufficient to raise the temperature of the dense phase of coal particles up to a temperature of about 400.degree. C upon exiting the externally heated pipelines.
The object of preheating the coal is to satisfy partially the enthalpy demand of the adiabatic-type hydrocarbonization reaction. Additional heat is supplied by the heat of reaction and by preheat added to process gases. The enthalpy demand consists of the heat required to raise the temperature of coal and process gas from their initial value to reaction temperature plus small heat losses. The actual temperature to which the coal feed must be preheated is, therefore, a function of the preheat added to process gases, and in the extreme may be ambient temperature i.e., zero preheat.
After being preheated to the desired temperature, the dense-phase flow of coal particles is reacted with hydrogen in the fluid-bed hydrocarbonization zone. Both agglomerating and non-agglomerating type coals may be employed in the continuous process of this invention without defluidization of the fluid-bed and plugging type problems. Agglomeration of coal particles in a fluid-bed may be substantially prevented by introducing solid coal particles into the fluid-bed hydrocarbonization zone at a high velocity.
Coal particles, especially caking, swelling or agglomerating coals become sticky when heated in a hydrogen-rich atmosphere. Even non-caking, non-swelling and non-agglomerating coals become sticky when heated in such an atmosphere. Coal particles begin to become sticky at temperatures in the range of about 350.degree. C to about 500.degree. C, depending on the specific properties of the coal, the atmosphere and the rate of heating. The stickiness results due to a tarry or plastic-like material forming at or near the surface of each coal particle, by a partial melting or decomposition process. On further heating over a time period, the tarry or plastic-like material is further transformed into a substantially porous, solid material referred to as a char. The length of this time period, generally in the order of minutes, depends upon the actual temperature of heating and is shorter with an increase in temperature. By plastic transformation as used throughout the specification is meant the hereinabove described process wherein surfaces of coal particles being heated, particularly when heated in a hydrogen atmosphere, develop stickiness and transform into substantially solid char, non-sticky surfaces. Plastic transformation is undergone by both normally agglomerating coals and coals which may develop a sticky surface only in a hydrogen-rich atmosphere.
Agglomerating or caking coals partially soften and become sticky when heated to temperatures between about 350.degree. C to about 500.degree. C over a period of minutes. Components of the coal particles soften and gas evolves because of decomposition. Sticky coal particles undergoing plastic transformation tend to adhere to most surfaces which they contact such as walls or baffles in the reactor, particularly relatively cool walls or baffles. However, contact with other sticky particles while undergoing plastic transformation results in gross particle growth through adherence of sticky particles to one another. The formation and growth of these agglomerates intereferes drastically with the maintenance of a fluid-bed and any substantial growth usually makes it impossible to maintain fluidization.
In particular, entrance ports and gas distribution plates of equipment used in fluid-bed coal conversion processes become plugged or partially plugged. Furthermore, even if plugging is not extensive, the sticky particles tend to adhere to the walls of the vessel in which the operation is conducted. Continued gross particle growth and the formation of multi-particle conglomerates and bridges interfers with smooth operation and frequently results in complete stoppage of operation.
Agglomeration of coal particles upon heating depends on operating conditions such as the heating rate, final temperature attained, ambient gas composition, coal type, particle size and total pressure. When heated in a hydrogen atmosphere, even non-agglomerating coals, such as lignites or coals from certain sub-bituminous seams, are susceptible to agglomeration and tend to become sticky in a hydrogen atmosphere. Thus, agglomeration of coal particles is accentuated in a hydrocarbonization reactor where heating in the presence of a hydrogen-rich gas actually promotes formation of a sticky surface on the coal particles reacted.
In a corresponding application filed concurrently herewith, "Method of Avoiding Agglomerating in Fluidized Bed Processes" by C. W. Albright and H. G. Davis, it is taught that agglomeration of a fluidized bed may be substantially prevented by introducing solid coal particles into a fluid-bed reaction zone at a high velocity. The fluid-bed is conventionally maintained by passing a fluidizing medium through finely-divided solid particles. "Introduction velocity" as used throughout the specification means the velocity of carrying gas through a device which causes the solids or liquid velocity to approach the maximum theoretical ratio to gas velocity, i.e., 1 to 1. By a high velocity is meant a velocity sufficient to rapidly and uniformly disperse fresh coal particles entering the fluid-bed at a temperature below the plastic transformation-temperature within a matrix of non-agglomerating particles in the fluid-bed. The non-agglomerating particles preferably are the hot, partially reacted coal particles and char particles situated within the fluid-bed reaction zone. Due to the difference of temperature between the entering coal particles and the reaction zone, the entering particles tend to agglomerate as heat transfers from the reaction zone to the entering coal particles. However, it has been found that when introduced in the fluid-bed at a high velocity, the entering coal particles rapidly and uniformly disperse within a matrix of non-agglomerating particles within the fluid-bed before being heated up to the plastic transformation-temperature range.
By this process, the entering, sticky or potentially sticky coal particles are rapidly distributed and brought into intimate association with non-sticky, hot particles situated within the fluid-bed reaction zone. The entering particles do not substantially adhere to the charry surfaces of these non-agglomerating hot particles which have passed through the plastic transformation-temperature range or are inherently non-agglomerating. The hot, non-agglomerating particles at bed temperature rapidly transfer heat to the entering coal particles causing them to transverse the plastic transformation-temperature range swiftly without contacting significant numbers of other sticky coal particles beforehand.
A velocity rate useful in the method of this invention may be obtained by any suitable means. For example, a narrow inlet port or any other inlet means which narrows or necks down the cross-sectional area of the passageway to the inlet where the fresh coal particles enter the reactor may be employed to accelerate the coal particles to a high velocity. In addition, process gas may be physically added to the fluidized stream of fresh coal at a point before the fluidized stream enters the reaction zone. The addition of process gas increases the flow rate of the fluidized stream and hence the velocity of the coal particles. An amount of process gas sufficient to achieve the desired entrance velocity of coal particles should be used.
Since the fluidized coal particles are transported through the lines in a dense phase flow, high velocity flow rates are usually unnecessary and undesirable due to the abrasive characteristics of coal. A high velocity, dense phase flow or coal particles throughout the lines would have required wear plates to be installed throughout the lines to control the otherwise rapid erosion rate of the lines, such wear plates being an undesirable expense. However, according to the present invention, only a small surface area will be exposed to abrasive wear and this part may be replaced readily and economically with little or no downtime of the system.
By maintaining the distance for transport between feeder and reactor minimal, a high velocity dilute phase or an intermediate dilute phase flow may be employed in the line or lines connecting the feeder and reactor. The increased wear and/or need for wear-resistant material is offset by the increased separation and dilution of coal particles on introduction into the reactor at a high velocity. This makes some additional contribution to avoiding agglomeration.
For example, an inlet means comprising a material having a wear-resistant surface may preferably be employed in this invention as a means for increasing the velocity of coal particles entering the reaction zone and as a means of controlling the manner of entry i.e., in a solid stream or in a fan-like uniform distribution. Use of such an inlet means lengthens the wear time of the surface exposed to the high erosion rate caused by the high velocity flow of coal particles.
Suitable wear-resistant surfaces would be composed of materials such as tungsten carbide, silicon carbide or other wear resistant materials known in the art in any combination or mixture thereof. For clarity and illustrative purposes only, the description of this invention will be mainly directed to the use of tungsten carbide as the wear-resistant surface of the material that reduces erosion in the lines although any number of other wear-resistant materials can be used successfully according to this invention.
An inlet means such as a nozzle which comprises a transfer line having a reduced or constricted cross-sectional area may be employed in the method of this invention. The length to cross-sectional area ratio of the nozzle should be sufficiently large enough so that the desired velocity of injection for the solid coal particles or non-vaporizable recycle oil may be achieved. A length to cross-sectional area of this section of transfer line of greater than about 5 to 1 is desirable, greater than about 10 to 1 preferable, and greater than about 20 to 1 more preferable. This allows for a finite distance which the coal particles and/or non-vaporizable recycle oil require for acceleration to the velocity approaching that of the carrying gas.
According to this invention, it is preferable to introduce a fluidized stream of coal particles into the lower end of a fluid-bed hydrocarbonization zone. More preferably the particles are introduced into the hydrocarbonization reactor through at least one inlet in the reactor in a vertically upwards direction. The inlet is situated substantially in the vicinity of the vertical axis at or near the reactor bottom. The coal particles are introduced at a velocity sufficient to mix the fresh coal having a temperature below the plastic transformation-temperature rapidly with non-agglomerating particles such as partially reacted coal and char particles in the reaction zone at the reaction temperature thereby substantially preventing agglomeration of the fluid-bed.
In the reactor which preferably is substantially vertical, the natural circulation of coal particles within the fluid-bed hydrocarbonization zone is a complex flow pattern. However, it may be described approximately by dividing the hydrocarbonization zone into two concentric sub-zones, an inner sub-zone and an outer sub-zone surrounding the inner sub-zones.
In the inner sub-zone which is situated substantially within the axially central portion of the reactor, coal particles flow in a generally ascending path. In the outer sub-zone, which is situated substantially near the walls of the reactor, coal particles flow in a generally descending path. Advantages of introducing the coal particles into the fluid-bed of the reactor in an essentially vertically upwards direction are that the natural circulation of coal particles in the fluid-bed is enhanced and the coal particles get at least a minimum residence time. Introduction of coal particles into the fluid-bed of the reactor promotes a channeled circulation of particles within the hydrocarbonization zone along the natural circulation path. Circulation eddies, are thus enhanced and promote the dispersion of the entering coal particles within the fluid-bed hydrocarbonization zone. Moreover, when introduced in this manner, the coal particles are ensured of not immediately and directly striking the sides of the vessel wherein the hydrocarbonization occurs, a result which could lead to unnecessary and undesirable agglomeration.
The fluidized coal particles should be introduced into this inner sub-zone, the central upflow zone within the reactor. The central upflow zone extends radially from the vertical axis of the reactor to an area where the outer-sub-zone, the peripheral downflow zone begins. It is essential that the coal particles be introduced into the central upflow zone in order to avoid striking the walls of the reactor or entering the peripheral downflow zone. Preferably, the coal particles are introduced through the base or bottom of the reactor at one or more inlets situated in the vicinity of the point where the vertical axis of the reactor intersects the base of the reactor.
It has been discovered that introducing a fluidized stream of coal particles into a dense phase, fluid-bed hydrocarbonization zone at a velocity of more than about 200 feet per second in a manner described hereinabove substantially prevents agglomeration or caking of the fluid-bed. When a lower injection velocity, for example, about 100 feet per second is used, agglomeration of the fluid-bed is not prevented. In order to substantially prevent agglomeration of the fluid-bed hydrocarbonization zone, coal should be introduced at a high velocity into the zone in a high velocity stream, i.e., at a velocity more than about 200 feet per second, and preferably more than about 400 feet per second in the manner described hereinabove.
The hydrocarbonization zone is maintained at an average temperature of from about 480.degree. C to about 600.degree. C by known heating methods. Preferably, a combination of preheat and heat of reaction is used to maintain the hydrocarbonization zone adiabatically at these temperatures. Although any convenient source of heat can be employed, it has been found that, when the feed coal is preheated in the range of about 250.degree. C to about 300.degree. C, preferably between about 250.degree. C and about 420.degree. C, the exothermic heat of reaction in the hydrocarbonization zone is sufficient to maintain the desired reaction temperature. It is also desirable to similarly preheat the process gas. Although temperatures of less than 480.degree. C can be employed, they are generally not desirable because the rate of reaction of coal with hydrogen is too slow for a practical process. The temperature must not exceed about 600.degree. C, however, for at these more elevated temperatures, several deleterious reactions occur during even the minimum residence times practicable in the kind of reaction described. Oxygen is more completely converted to water and carbon oxides, heavy liquid products are converted to coke and lighter liquids and gases, and hydrogen consumption increases.
The temperature of reaction may be controlled at a desired point within the operating range of 480.degree. C to 600.degree. C by choice of and control of the preheat conditions. Temperatures in the range of 520.degree. C to 580.degree. C are preferred.
The gas employed can be pure hydrogen or hydrogen in admixture with an inert gas such as nitrogen or the like. For the purpose of this reaction, recycle gases such as methane and ethane may be considered to be essentially inert. However, the hydrogen partial pressure in the hydrocarbonization zone should be between about 100 p.s.i. to about 1200 p.s.i. At partial pressure of less than 100 p.s.i. the rate of reaction with the coal is too slow, and at partial pressures of greater than 1200 p.s.i. the amount of hydrogen consumed is too great for an economical process and the difficulties of avoiding agglomeration become too great for a practical and economical process. Hydrogen partial pressures of from about 300 to about 500 p.s.i. are preferred, and from about 200 to about 800 p.s.i. desirable. By the term "hydrogen partial pressure", as employed in the specification and claims, is meant the log mean average of the hydrogen partial pressure in the feed and product gas streams.
The coal, when it is fed into the hydrocarbonization zone, is rapidly hydrocarbonized, leaving a solid particulate char in the bed, which is then withdrawn from the bed. The coal is fed at a rate such that the average solid residence time in the hydrocarbonization zone is from about 1 to 30 minutes, preferably 3 to 12 minutes, and may be from about 5 to about 60 minutes, more preferably about 8 to about 30 minutes. By the term solid residence time as employed in the specification is meant the time needed to fill the empty reaction zone with reacting coal. It is calculated by multiplying the volume of the reacting zone by a fluid-bed density of coal per unit volume typically 30 to 38 pounds per cubic foot, and dividing this by the coal feed rate.
In the hydrocarbonization zone, the coal is converted to hydrocarbon-rich gas, to oil and to char in proportions which can be varied by varying temperature, time and pressure. Preferably, char yield is the minimum amount sufficient to make the hydrogen and plant fuel. Make char may be dropped through an overflow pipe to valved hot receivers, which may be intermittenly depressurized and dumped. Entrained char may be removed from the vapor overhead by cyclones or the like and returned to the bed. Oil, water and gas products may be separated by methods well known in the art such as staged condensation and the like.
As indicated above, it is the object of this invention to maximize the amount of liquid products, particularly phenolic compounds, including phenols, cresols, xylenols, ethyl phenols and the like in proportion to the amount of hydrogen consumed. It is also the object of this invention to maximize the amount of liquid and gaseous fuel products in proportion of the amount of hydrogen consumed. It has been discovered by this invention that, to obtain these objectives, not only must the process conditions be maintained within the limits set forth above, they also depend upon each other.
This interdependence of process conditions results because of the effect of each variable on product yields and hydrogen consumed. For example, the yield of tar increases with increasing time, but tends to level off at a limit, which is dependent on temperature and pressure, above which there is little or no increases in tar yield. This limit is about 10 minutes at above 540.degree. C, and may be, at times, about 15 minutes at about 540.degree. C, but decreases to as low as 8 minutes at 570.degree. C. The precise limit will vary, of course, depending on the particular coal or hydrogen partial pressure employed. On the other hand, the amount of hydrogen consumed increases continuously with increasing residence time. Both the yield of tar and the amount of hydrogen consumed increase with increasing hydrogen pressure, with the amount of hydrogen consumed increasing proportionately faster.
Other desirable product mixes may be obtained by selecting appropriate operating conditions. For example, product gas yields may be increased at the expense of higher hydrogen consumption at a constant residence time and hydrogen partial pressure. As temperature increases as described above, liquid yield first increases, then reaches maximum and decreases. Gas product may be increased at the expense of liquid product by recycling all or part of the latter. On the other hand, at mild operating conditions wherein hydrogen consumption is less than about 2 percent of the weight of the MAF coal, liquid yield will be high when compared to gas yield.
The variable having the greatest effect on tar and phenolic yield per unit of hydrogen consumed is temperature. In general, the yield of tar increases slowly with temperature to a maximum, and then decreases because of hydrocracking of tar components to uncondensable gases and coke. On the other hand, the amount of hydrogen consumed increases very rapidly with temperature. In addition, the amount of oxygen in the char decreases with temperature and, once the char is depleted of oxygen, further reaction of char and hydrogen will not produce the desired phenolic products. As a result, the yield of tar and phenols per unit of hydrogen consumed remains high up to a maximum temperature dependent on residence time and pressure and then rapidly decreases.
Accordingly, to maximize the yield of phenols and tar per unit of hydrogen consumed, the process variables must conform to the relationship defined by the equation: EQU S.sub.H = T(P).sup.0.067 (t).sup.0.067 (I)
wherein S.sub.H is the hydrocarbonization severity factor having a value of from 530 to 640, preferably from 560 to 630; T is the average hydrocarbonization temperature in .degree.C; P is the log mean average hydrogen partial pressure in p.s.i. divided by 1000; and t is the solids residence time in minutes. When this relationship is observed, the weight ratio of phenolic compounds boiling below 230.degree. C to hydrogen consumed will generally be about 3.5 to about 5 or higher. This is true for the preferred class III and IV coals. Furthermore, to maximize yield of total liquid products, the operating range for S should be 550 to 700, preferably 600 to 680.
Many products produced by the hydrocarbonization of coal in accordance with this invention are cresols and other substituted phenols which may desirably be dealkylated to form phenol. Although the dealkylation can be accomplished in a step separate from the hydrocarbonization, it has been found by this invention that, if the vapors produced by the hydrocarbonization are retained in the fluidized bed for from about 10 seconds to about 250 seconds preferably from about 30 to about 150 seconds, considerable dealkylation of the substituted phenols occurs. It has been further found that the presence of the char in the fluidized bed acts as a catalyst for the dealkylation permitting a degree of dealkylation equivalent to that obtained at higher temperatures in the absence of the char.
For optimum results from this dealkylation step, it has been found that the temperature and vapor residence time must conform to the following equation: EQU S.sub.c - T(.theta.).sup.0.048 (II)
wherein S.sub.c is cracking severity factor having a value of from 640 to about 750, preferably 650 to 710; T is the temperature in C; and .theta. is the vapor residence time in seconds. In other circumstances, when it is preferable to maximize total liquid product and minimize hydrogen consumption, it may be desirable to operate at the lowest practical range of S.sub.c, from 600 to 690.
The product gas comprises vapor products from the hydrocarbonization and consists mainly of gaseous products such as water, carbon dioxide, carbon monoxide, methane and the like, e.g. other hydrocarbons, as well as unreacted hydrogen, and condensable tar fraction. The tar fraction can be readily distilled to recover valuable chemicals, including phenols. The tar contains a sizeable quantity of material boiling at temperatures in excess of about 230.degree. C which is useful mainly as a fuel. It has been found by this invention, however, that this high boiling material can be recycled to the hydrocarbonization zone to be hydrocracked to compounds boiling below 230.degree. C, thereby permitting substantially all of the vapor products produced by the hydrocarbonization to be recovered as valuable, low-boiling chemicals. The high-boiling material is fed to the hydrocarbonization zone at a point sufficient to permit conversion of about 25 to 40 percent of the recycled materials to products boiling below about 230.degree. C. In this manner the over-all yield of low boiling phenolic materials is increased and the ratio of phenols produced to hydrogen consumed is also increased. Because this material will flash vaporize when fed to the hydrocarbonization zone, the hydrocarbonization is still conducted in the dry phase. By this modification, the three process steps of hydrocarbonization, dealkylation of substituted phenols, and a secondary hydrocracking of coal tars are conducted simultaneously.
It has also been found by this invention that this tar or fraction of it may be recycled to the hydrocarbonization reaction and thereby converted to lower boiling liquids, gases and char. Such recycle can be accomplished without agglomerating the fluid-bed if the recycle liquid is injected at a sufficiently high velocity as described above to admix it rapidly with partially reacted coal and char particle and at such an angle as to avoid its sticking to the walls or intervals of the bed before reacting or vaporizing.
The char produced by the hydrocarbonization process of this invention is very reactive and contains fairly large quantities of hydrogen, generally about 4 weight per cent of an MAF basis. It has been found that if the char is heated to a temperature of 800.degree. to 900.degree. C or higher, preferably 840.degree. to 800.degree. C, one can obtain a gas stream containing about 75 to 85 volume per cent hydrogen, with the balance comprising mainly carbon monoxide and methane. This hydrogen stream, after removal of the contaminants, can be employed as the fluidizing gas, thereby substantially lessening the requirement for hydrogen from some other source. It is preferred that this "calcination" process be conducted in a fluidized bed, employing, for example steam as the fluidizing medium. The pressure is preferably atmospheric. The solid residence time in the calcination zone can vary from about 2 to about 10 minutes, and is preferably from about 3 to about 7 minutes. When steam is the fluidization gas, partial reaction occurs augmenting the hydrogen and carbon monoxide yields. It is not intended to restrict the method of using the char to this process, however, for the char is suitable as feed to any of several commercial or proposed gasification processes.